1. Field of the Invention
The present invention relates to an electronic switch circuit including a contactless switch composed of transistors, and more specifically to improvement of an electronic switch circuit used as a part of an electronic change-over device which is incorporated in for example a television receiver and a video tape recorder.
2. Description of Related Art
Recent television receivers and video tape recorders have been equipped with electronic change-over devices for controlling the conditions of a displayed image and an audio output, and these electronic change-over devices are adapted to be operated by for example a remote control device.
Most of electronic switch circuits used in the electronic change-over devices are composed of transistors so as to utilize an ON/OFF operation of a transistor which is obtained by changing a base-emitter voltage. For example, in a television receiver, switching signals of different levels corresponding to an audio output condition and a muting condition, respectively are selectively applied to a base of a transistor, so that the transistor is on/off-controlled dependently upon the level of the switching signal, with the result that an audio signal is selectively either outputted or muted.
Referring to FIG. 1, there is shown a circuit diagram of a typical conventional electronic switch circuit. The shown electronic switch circuit includes a pair of PNP transistors Q.sub.1 and Q.sub.2 having their commonly connected emitters which are connected through a constant current source I.sub.1 to a high voltage supply line Vcc. A base of the transistor Q.sub.1 is connected to an emitter of a PNP transistor Q.sub.3, which has a base connected to a reference voltage V.sub.1 and a collector connected to the ground GND. A collector of the transistor Q.sub.1 is connected to a collector of an NPN transistor Q.sub.4 having an emitter connected to the ground GND.
The electric switch circuit also includes a pair of NPN transistors Q.sub.5 and Q.sub.6 having their commonly connected collectors which are connected to the high voltage supply line Vcc. A base of the transistor Q.sub.5 is connected to the collector of the transistor Q.sub.1, and an emitter of the transistor Q.sub.5 is connected to a base of the transistor Q.sub.4 as well as a base of a switching transistor (not shown) incorporated in for example an external amplifying circuit (not shown).
A base of the transistor Q.sub.2 is connected to an emitter of a PNP transistor Q.sub.7, which has a base connected through a resistor R.sub.1 to an input terminal S.sub.1 and a collector connected to the ground GND. A collector of the transistor Q.sub.2 is connected to a collector of an NPN transistor Q.sub.8 having an emitter connected to the ground GND. A base of the transistor Q.sub.6 is connected to the collector of the transistor Q.sub.2, and an emitter of the transistor Q.sub.6 is connected to a base of the transistor Q.sub.8 as well as a base of another switching transistor (not shown) incorporated in the above mentioned external amplifying circuit (not shown).
In the above mentioned electronic switch circuit, if a voltage larger than the reference voltage V.sub.1 (a high level signal H) is applied to the input terminal S.sub.1, the transistor Q.sub.1 is turned on and the transistor Q.sub.2 is turned off, so that the transistor Q.sub.5 outputs an control signal so as to turn on the first transistor of the not-shown external amplifying circuit.
On the other hand, if a voltage smaller than the reference voltage V.sub.1 (a low level signal L) is applied to the input terminal S.sub.1, the transistor Q.sub.1 is turned off and the transistor Q.sub.2 is turned on, so that the transistor Q.sub.6 outputs another control signal so as to turn on the another transistor of the not-shown external amplifying circuit.
In bipolar transistors, when the transistor is on, a voltage difference corresponding to one diode will occur between a base and an emitter of the transistor. In the above mentioned electronic switch circuit, therefore, if an input signal of zero volt is inputted to the input terminal S.sub.1 as the low level signal (L), a forward bias is applied between the base and the collector of the transistor Q.sub.2, and therefore, the transistor Q.sub.2 becomes difficult to turn on.
Assuming that a base-emitter voltage drop V.sub.BE is 0.7 volts, if the input signal of zero volt is inputted to the input terminal S.sub.1, the collector of the transistor Q.sub.2 is brought to 1.4 volts because of the turned-on transistors Q.sub.6 and Q.sub.8 and the emitter of the transistor Q.sub.2 is brought to 0.7 volts because of the turned-on transistor Q.sub.7, so that a forward bias of 0.7 volts is applied between the base and the collector of the transistor Q.sub.2. Therefore, it is necessary to apply a voltage lower than the reference voltage V.sub.1 to the input terminal S.sub.1 in order to cause the transistor Q.sub.6 to output the control signal, but, it is not a sufficient condition. Namely, under the above assumption, the voltage applied to the input terminal S.sub.1 must be of 0.7 volts or more. Incidentally, this voltage is dependent upon a material of the transistor. In any case, a range of the switching voltage is limited. In addition, the transistors Q.sub.3 and Q.sub.7 can be omitted in the circuit shown in FIG. 1. In this case, however, the low level input voltage must be not smaller than 1.4 volts.
In general, the television receivers are such that an output signal outputted from a microcomputer is applied as the switching signal to the input terminal S1. However, the output signal of the microcomputer is for example 0 volt or 5 volts, and therefore, 0 volt is assigned to the low level signal. As mentioned above, if a signal of 0.7 volts or more is required as the low level signal, a new limitation is placed on a design of the output circuit of the microcomputer.